Implantable stents and other expandable medical devices have been increasingly used in many minimally invasive surgical procedures such as balloon angioplasty. Stents mitigate complications of acute and subacute vessel closure, intimal dissection, and elastic recoil of the vessel wall, and reduce angioplasty-related restenosis rates. Restenosis can be caused by incompatibility of the metallic surface of the stent that engages the inner walls of the blood vessel, giving rise to subacute thrombosis. Another possible cause of restenosis is the recoil of the metallic surface of the stent when placed along the internal wall of the artery. In-stent restenosis usually occurs within weeks to months of stent implantation.
Some metallic devices such as vascular stents, made from stainless steel or Nitinol alloys (Shape Memory Alloys, such as Nickel Titanium alloy), may undergo corrosion upon long-term implantation. The corrosion products such as transition metal ions may be toxic to the surrounding tissue. To prevent corrosion, stents or stent-grafts have been coated with a polymeric or biological material. However, when coated stents are expanded within a blood vessel, the coefficient of expansion of the coating greatly differs from that of the expanding stent such that, upon expansion, the surface of the coating tears rendering the expanded stent uncoated. If a thicker coating is applied, such a thick coating can deter stent expansion or can render implantation into the body difficult. Thus, challenges remain in developing biostable metallic implantable devices.